Multiple-site wireless communication systems are well known in the art. In such systems, each site gives rise to its own wireless coverage area. By strategically locating the multiple sites, apparently seamless wireless coverage is provided to communication units (e.g., in-hand portable or in-car mobile radios) over an area significantly larger than the coverage area corresponding to a single site. At any time within such systems, a communication unit is affiliated with a serving site. A serving site is that site currently used to support communications with the communication unit. Any sites adjacent to the serving site (i.e., those sites which have at least some overlapping coverage area with the serving site) are considered to be neighboring sites. Throughout normal operation, the communication unit constantly determines whether the quality of service in one of the neighboring sites is sufficiently better than the quality of service in its current serving site. When this happens, a new serving site is selected, likely giving rise to an entirely new set of neighboring sites.
In determining qualities of service, each communication unit typically monitors outbound channels corresponding to its serving site and at least one of the neighboring sites. In the context of the present invention, an outbound channel refers to a wireless channel transmitted by a site and received by communication units. If a given outbound channel is being received well (strong received signal strength, relatively infrequent received errors, etc.) the quality of service is deemed good. Conversely, if the outbound channel is being received poorly (weak received signal strength, relatively frequent received errors, etc.) the quality of service is deemed bad. Often, the outbound channels monitored by the communication unit are control channels continuously transmitted by each of the sites, as known in the art.
An example of such monitoring is shown in FIG. 1, which illustrates a prior art method for monitoring a given outbound channel. In particular, FIG. 1 illustrates a time-division multiplexed (TDM) outbound channel in which samples 101.107 are periodically taken by a communication unit. Such samples are taken while the communication unit is in standby mode, i.e., not engaged in a communication. However, the sampling period (or, reciprocally, the sampling rate) is static and cannot be changed regardless of how well, or poorly, the outbound channel is being received. For example, in current "IDEN" systems, by Motorola, Inc., a communication unit must sample neighboring sites every 180 milliseconds and its serving site every 360 milliseconds.
As a result of this constant monitoring, a significant current drain is placed on the communication units. This current drain represents a substantial performance limitation on portable units where battery capabilities are limited. Using the sample periods given above, the battery charge of a typical portable unit is expended after approximately six hours. To combat this problem, it is known in the art to allow communication units to change, to a limited extent, the sampling period used when monitoring outbound channels. For example, in a system adhering to Global System for Mobile Communications (GSM) type standards, a communication unit can alter the sampling period used when monitoring outbound communication channels. (A description of the "Discontinuous Reception" mode in GSM systems can be found in the ETSI-GSM Technical Specification: GSM 03.13 version 4.01 dated Jul. 1, 1993, European Digital Cellular Telecommunications System (Phase 2).) As a unit roams, it is instructed by its various serving sites what sampling period (or rate) to use. The sampling rates sent to the communication units are based on known factors derived when the system was originally configured, e.g., the quality of coverage throughout a given site. Although this method does allow for dynamic sampling periods, the individual circumstances of a communication unit are not reflected in the sampling periods used. As a result, the maximum potential battery life for any given communication unit is not likely to be realized. Therefore, a technique that allows the circumstances of a given unit to control the sampling period used to monitor outbound channels would represent a significant advancement over the prior art.